Discrete multitone modulation (DMT), also referred to as multicarrier modulation, is a modulation method which is particularly suitable for transmitting data via channels in which linear distortion occurs. In comparison to so-called single-carrier methods such as amplitude modulation, which has only one carrier frequency, discrete multitone modulation makes use of a is large number of carrier frequencies. The amplitude and phase of each individual carrier frequency is modulated using quadrature amplitude modulation (QAM). This thus results in a large number of QAM-modulated signals. A specific number of bits may in each case be transmitted per carrier frequency. Discrete multitone modulation is used for digital audio broadcast DAB where it is referred to as OFDM (Orthogonal Frequency Division Multiplex) and for transmitting data via telephone lines, where it is referred to as ADSL (Asymmetric Digital Subscriber Line).
In ADSL, a DMT-modulated signal is used to transmit data from a switching center via a subscriber line to a subscriber with an analog connection. In this case, ETSI and ANSI Standards state that each carrier frequency has a bandwidth of approximately 4 kHz, and that at most up to 15 bits per second per Hz are transported. The actual number of bits per second per Hz may differ for each carrier frequency, thus allowing the data rate and transmission spectrum to be matched to the transmission channel.
A DMT transmission system has a coder which combines the bits in a serial digital data signal which is intended to be transmitted, to form blocks. A specific number of bits in a block in each case have an associated complex number. A complex number is used to represent a carrier frequency f1=i/T where i=1, 2, . . . , N/2 in the discrete multitone modulation, with all the carrier frequencies fi being distributed at equal intervals. T is the time duration of a block. Inverse Fourier transformation is used to transform the carrier frequencies represented by the complex numbers to the time domain, where they directly represent N samples of a DMT signal to be transmitted. In order to allow Inverse Fast Fourier Transformation (IFFT) to be used, a power of two is selected for N. This reduces the complexity for Inverse Fast Fourier Transformation.
After the Inverse Fast Fourier Transformation, a cyclic prefix is carried out, with the last M (M<N) of the samples being attached once again to the start of a block. A periodic signal is thus simulated for a receiver, once the transient process produced by a transmission channel has decayed after M samples corresponding to a time T·M/N. The equalization complexity in the receiver can be greatly reduced by means of the cyclic prefix since, after demodulation in the receiver, all that is necessary is multiplication by the inverse of the transfer function of the transmission channel in order to compensate for the linear distortion in the transmission channel. This requires one complex or four real multiplications for each carrier frequency.
In ADSL, the physical transmission channel is a two-wire line (twin-core copper cable) in the telephone network. The two-wire line requires a long time for the transient process in comparison to the length of a block. On the other hand, any additional transmission capacity required as a result of the cyclic prefix is intended to be as low as possible.
A cyclic prefix of M=32 is defined in ADSL for a block length of N=512. However, the transient process on the two-wire line has not yet decayed after M=32 values. Additional errors thus occur in the receiver, which cannot be compensated for by a frequency-domain equalizer.
Such additional errors can be reduced by using special signal processing measures in the receiver.
To this end, a time domain equalizer (TDEQ) is connected upstream of a demodulator. The time domain equalizer is in the form of a digital transversal filter, whose coefficients are adjustable. The object of the time domain equalizer is to shorten the transient process of the transmission channel. The design of such time-domain equalizers is described in Al-Dhahir, N., Cioffi, J. M., “Optimum Finite-Length Equalization for Multicarrier Transceivers”, IEEE Trans.on Comm., Vol. 44, No. 1, January 1996. However, this has the disadvantage that the digital transversal filter used as the time-domain equalizer has a large number of coefficients, and the adaptation of the digital transversal filter is complex. A filter length of 20 to 40 coefficients means that approximately 50 to 100 million multiplication operations must be carried out per second. In addition, each coefficient must be adjusted for adaptation of the digital transversal filter.
The technical problem on which the invention is based is thus to specify a digital receiver for a signal produced using discrete multitone modulation, which receiver has a time-domain equalizer which can be adapted more quickly and which carries out fewer multiplications per second.